Radiation induced crosslinking reactions of functionalized monomers, oligomers, and polymers play an important role in a variety of commercial applications extending from the ultraviolet curing of varnishes and inks to the imaging of semiconductor chips. There is a need for high temperature resists, permanent dielectrics or solder masks having improved mechanical properties.
Recently several classes of cationic photoinitiators have been found and their use in combination with epoxies is described by G. E. Green, B. P. Stark, and S. A. Zahir, in J. Macromol. Sci. Revs. Macromol. Chem., C21, 187 (1981/82). However, many of these initiators are slow and inefficient and thus have limited practical value. Commercially, the most significant catalysts are the aryldiazonium, triphenylsulfonium and diphenyliodonium salts, with the most recently found compounds the diaryliodosyl and triarylsulfoxonium salts which possess anions of low nucleophilicity. These are known to liberate, upon irradiation, the corresponding Lewis acids BF.sub.3, BF.sub.5 etc., or the Bronsted acids HB.sub.4, HPF.sub.6, HAsF.sub.6 etc., shown below:
A. Diazonium salts
hv EQU ArN.sub.2.sup.+ MX.sub.n.sup.- .fwdarw.ArX+MX.sub.(n-1) +N.sub.2(I) PA1 hv EQU Ar.sub.p Z.sup.+ MX.sub.n.sup.- +HY.fwdarw.Ar.sub.(p-1) Z+Ar.sup.+ +Y.sup.- +HMX.sub.n (II)
B. Diaryliodonium and Triarylsulfonium salts
HY=H-donor PA0 MX.sub.n.sup.- =BF.sub.4.sup.-, PF.sub.6.sup.-, AsF.sub.6.sup.-, SbF.sub.6.sup.- etc. PA0 Z=iodine or sulfur
The effectiveness of aryldiazonium salts as photoinitiators depends on the structure of either the cationic or the anionic moieties of these salts. Their spectral sensitivity can be varied by modifying the structure of the aryl part of the aryldiazonium compound. Generally when utilizing this type of initiator a thermal post treatment step is required after irradiation to achieve satisfactory cure of the epoxy resin. However, there are several drawbacks that limit the use of aryldiazonium salts as photoinitiators in many practical applications such as nitrogen evolution during the photolysis step, poor thermal stability and inherent moisture sensitivity.
Crivello in "Developments in Polymer Photochemistry 2" (N. S. Allen, ed.), P. 1 Applied Science Publ., London, 1981, postulated that the reaction mechanism involves homolytic cleavage of one of the aryl bonds induced by a photochemical reaction is the first reaction step, then a subsequent hydrogen abstraction from a suitable donor, followed by loss of a proton yields the Bronsted acid HX. This strong Bronsted acid HX, protonates the oxirane group as the initial step with subsequent ring opening polymerization taking place. Cycloaliphatic epoxies show higher reactivities than the glycidyl ethers and glycidyl esters. Most of the examples reported to date involve organometallic complexes that possess photolabile ligands such as carbon monoxide, olefins, and carbocyclic rings, as described by D. M. Allen, J. Photog. Sc., 24, 61 (1976) and H. Curtis, E. Irving, B. F. G. Johnson, Chem. Brit., 22, 327 (1986).
Numerous organometallic compounds, such as organometal carbonyl compounds, metallocenes and aluminum complexes can act as photoinitiators in the polymerization of epoxy functionalized polymers. K. Meier and H. Zweifel, J. Imag. Sci. 30, 174 (1986) have described iron arene salts having anions with low nucleophilicity. The photolysis of these compounds produces Lewis acids which can polymerize epoxy resins relatively easily. The development of insoluble networks requires a thermal activation step after the exposure step. Iron arene salts are generally prepared from the ferrocene according to the method reported by N. A. Nesmeyanov et al., Dokl. Akad. Nauk SSSR, 149, 615 (1963) as shown in FIGS. 1 and 2. Epoxide polymerization with iron arene complexes is exemplified in FIG. 3. During the photolytic process a ligand-exchanged iron complex having three coordinated epoxide functionalities is proposed to form. The ring opening polymerization of the epoxide is proposed to start in the ligand sphere of the iron cation as shown in FIG. 3.
An alternative to epoxy resin based materials are cyanate resins. Cyanate resins are more desirable compared to epoxy resins as resist materials due to their greater thermal and dimensional stability, better dielectric properties, and outstanding adhesive properties.
Cyanate ester resins are made from polyfunctional cyanate monomers as described in U.S. Pat. No. 4,094,852. Generally, a catalyst is employed to achieve lower curing temperatures and faster curing times. Thermally activated catalysts which are effective include acids, bases, salts, nitrogen and phosphorous compounds, i.e., Lewis acids such as AlCl.sub.3, BF.sub.3, FeCl.sub.3, TiCI.sub.4, ZnCl.sub.2, SnCl.sub.4 ; Bronsted acids such as HCl, H.sub.3 PO.sub.4, aromatic hydroxy compounds such as phenol, p-nitrophenol, pyrocatechol, dihydroxynaphthalene; various other compounds such as sodium hydroxide, sodium methoxide, sodium phenoxide, trimethylamine, triethylamine, tributylamine, diazabicyclo2.2.2!octane, quinoline, isoquinoline, tetrahydroquinoline, tetraethylammonium chloride, pyridine-N-oxide, tributylphosphine, zinc octoate, tin octoate, zinc naphthenate, and mixtures thereof U.S. Pat. No. 3,694,410 discloses the use of chelates of metal ions of the ionic or nonionic type with 1 to 6 or more chelate rings, to catalyze the formation of triazines from aromatic polyfunctional cyanates. Similarly, U.S. Pat. No. 4,528,366 has shown that cobalt salts of C.sub.6-20 carboxylic acids are useful catalysts for triazine formation, preferably cobalt octoate and cobalt naphthenate. U.S. Pat. Nos. 4,604,452 and 4,608,434 disclose that alcoholic solutions of metal carboxylates are effective catalyst compositions for triazine formation by heating. Organometallic cobalt compounds have been used to catalyze the trimerization of acetylenes as described in U.S. Pat. No. 4,183,864 and the co-trimerization of acetylenes and nitriles as described in U.S. Pat. No. 4,328,343. The photocatalyzed trimerization of aryl isocyanates using metal carbonyl complexes has been disclosed by E. Martelli, C. Pellizzi, and G. Predieri, J. Molec. Catalysis 22, 89-91 (1983). Energy polymerizable compositions comprising ionic salts of organometallic complex cations and cationically sensitive materials and the curing thereof has been taught in European Patent Nos. 109,851; 094,914; and 094,915. U.S. Pat. No. 4,554,346 discloses photocurable resins from cyanate ester compounds. The inventors used mixtures of polyfunctional cyanate esters with at least one compound having hydroxy groups and radical polymerizable unsaturated double bonds, the compounds used in quantities such that the ratio of cyanate groups to the hydroxy groups is in the range from 1:0.1 to about 1:2, and a radical polymerization photoinitiator, at elevated temperature. These materials would not be expected to yield the same polytriazine materials which can be obtained from direct polymerization of the cyanates only, and would be expected to lack the high thermal stability of the current invention.
Recently, Pujol et al. (U.S. Pat. No. 5,143,785) disclosed an energy-curable one-part adhesive composition comprising of the reaction product of an admixture of a cyanate ester resin, a thermoplastic polymer and a catalyst which can be thermally or photochemically activated. This composition was designed for fabricating adhesive films for bonding of electronic components such as semiconductor or integrated circuits to circuit boards or substrates. Their preferred adhesive composition included a silane coupling agent and may contain electrically conductive particles to enhance thermal conductivity between the die and substrate. This composition was not intended for forming dielectric patterns or as a resist material. The addition of the thermoplastic polymer improved the film-forming properties of these adhesives but was not correlated to phase separation or the effect of composite morphology on material properties.
Gelorme et al. (EP 0143087A1) disclosed a photoresist composition containing a curable cyanate ester resin or prepolymer and a cationic photoinitiator with non-ethylenic unsaturated monomers or polymers. These compositions were directed for use as photosensitive materials that could act as positive or negative resists. Patterns of the compositions were formed by exposing the photoresist material to actinic light (e.g., X-rays, ultraviolet radiation or electron beam radiation) through a mask to image and selectively crosslink the exposed areas. These photoprocessable cyanate resin based materials offered the benefits of good resolution and greater temperature stability than photosensitive epoxy resin based materials. The later is particularly advantageous for use as permanent resist of solder masks layers which typically experience elevated temperatures (up to 360.degree. C.) during soldering or chip joining operations. In addition, these formulations could be made to be positive or negative acting resists, depending on the type of photoinitiator selected. However, these compositions possessed brittle characteristics inherent with cured cyanate thermosets which resulted in marginal mechanical properties.
U.S application Ser. No. 07/923,723, filed on Jul. 31, 1992, the teaching of which is hereby incorporated by reference, described a conventionally processed, energy curable cyanate resin or prepolymer having incorporated therein a reactive, fluorinated thermoplastic oligomer modifier to reduce brittleness. The electrical and mechanical properties of the composition, such as fracture toughness, dielectric constant, coefficient of thermal expansion, flame retardancy, moisture uptake and glass transition temperature could be selected within value ranges. Conductivity of heat and of electricity also could be selected, by loading with a judicious choice and amount of organic or metal particles.
The reduced brittleness was attributed to the dissolution of the thermoplastic oligomer modifier and the thermoset cyanate resin to form a separation of the thermoset and thermoplastic during curing into submicron scale phases.
The reactive cyanate oligomers embodied in the invention are taught in U.S. application Ser. No. 07/923,723, as are the thermoplastic, fluorosubstituted modifiers, the formation of the microphases, fillers and workable and preferred concentration ranges. Although the final product possessed an excellent combination of thermal, electrical and mechanical for a variety of uses, it was not photoimageable. The present invention renders such compositions photoimageable for applications requiring improved thermal and mechanical properties.